Chain transmissions, in which a chain is engaged with a driving sprocket and one or more driven sprockets, have been widely used as timing transmissions in automobile engines for driving valve-operating cam or cams from the engine crankshaft.
Recent demand for higher power automobile engines, coupled with public consciousness of environmental problems, has led to the development of engines that produce high levels of noise and vibration, and to efforts toward reduction of that noise and vibration. The principal source of timing transmission noise is the engagement sound generated as the chain engages the sprockets. Attempts have been made to reduce noise and vibration by utilizing vibration-proofing materials to absorb radiated sound. For example, as shown in FIG. 10, a sprocket 40 is provided with an annular elastic member 44 sandwiched between an inner circumferential member 46, which has a keyed hole 46a by which the sprocket is fixed to a shaft, and an outer circumferential member 42. on which the sprocket teeth are formed. Examples of sprockets having sandwiched elastic members are shown in Japanese Laid-Open Utility Model Publication No. Sho. 59-35765, and in Japanese Laid-Open Patent Publication No. Hei. 9-264400.
When the sprocket 40 engages with a standard roller chain 60, as shown in FIG. 11, and the sprocket rotates clockwise, a following roller 62 moves relative to the sprocket in an arc centered on the center O1 of a preceding roller 62, which has been seated on a tooth gap bottom. The arc has the chain pitch p as its radius. Accordingly, the following roller moves in its arcuate path relative to the sprocket, and collides with a tooth gap bottom, near the center thereof, substantially at a right angle. The kinetic energy of the roller is transmitted to the tooth gap bottom without being buffered at the beginning of engagement. As a result, a large engagement impact is transmitted to the sprocket shaft. The performance of the elastic member 44 in shutting out vibration has been found to be insufficient. Moreover, when excessive impact force is applied to the elastic member 44, the endurance of the elastic member 44 is reduced.
Since the chordal tooth form pitch pa of the sprocket 40 is equal to the pitch p of a standard roller chain 60, the respective following rollers 62 abut the teeth of the sprocket 40 at the same abutment position t, as shown in FIG. 11. The abutment position in each case is at the point intersection of a radial center line X and a tooth gap bottom. Therefore, the engagement of a roller or bushing always engages the sprocket 40 at the same engagement point, and vibration and noise having an order determined by the number of sprocket teeth are increased. The elastic member has been found to be incapable of reducing these noises and vibrations adequately.
The standard roller chain shown in FIG. 11 is a power-transmitting roller chain defined by the Japanese Industrial Standards (JIS) or by the International standards (ISO), and has a uniform chain pitch p (The distance between the centers O1 of the respective rollers 62). A standard bushing chain may be used in place of the standard roller chain 60. In such a case, the elements 62 in FIG. 11 can be regarded as bushings.
Accordingly, an object of the invention is to provide a chain transmission in which a roller of a standard roller chain, or a bushing of a standard bushing chain, engage with a sprocket tooth, in which the vibration-reducing performance of an elastic member incorporated into the sprocket is improved, and in which the endurance of the elastic member is improved